Ensemble Of Quantum Dots Research Articles

Controlled storage and transfer of photonic space-time quantum-coherence in active quantum dot nanomaterials

We demonstrate the potential of semiconductor quantum dot nanomaterials for solid-state based controllable quantum memories in which losses may be compensated by gain. The dynamic photonic quantum-coherence present in a quantum dot ensemble and generated by a coherent signal pulse is influenced and controlled by disorder, spectral detuning and the power of the pulse. We show that the high coupling of spatial and temporal degrees of freedom is a key requirement for coherence transfer and/or storage.

Mean-field theory of Coulomb blockade distribution for a disordered ensemble of quantum dots

Techniques for synthesizing quantum dots (QDs) result in an assembly in which a certain amount of disorder is inevitable. Experimental probes often record the collective properties of an assembly of QDs and not exclusively that of a single QD. On the other hand, theoretical calculations often limit themselves to an analysis of a single QD. In the present work we present a mean field theory for Coulomb blockade (CB) in an assembly of QDs. We consider two types of disorder: (i) Size disorder; e.g., QDs have a distribution of sizes which could be unimodal or bimodal in nature. (ii) Potential disorder with the confining potential assuming a variety of shapes depending on growth condition and external fields. We illustrate our methodology assuming a Gaussian or logarithm normal distribution in disorder in both size and potential. However, our theoretical framework can accommodate any experimentally provided distribution in disorder. To do this we rely on the scaling laws for CB (also termed as Hubbard U) obtained for an isolated QD. We observe that CB is partially suppressed by the disorder. This suppression is greater for bimodal distribution. Further, the distribution in $U$ is a skewed Gaussian with enhanced broadening. We invoke the Lifshitz argument to analyze the bimodal case. We suggest how this distribution in CB can be experimentally studied.

Intermixing during growth of InAs self-assembled quantum dots in InP: A photoluminescence and tight-binding investigation

Atomic intermixing during the growth of self-assembled InAs quantum dots (QDs) in InP(001) by chemical beam epitaxy (CBE) can result in the formation of graded interfaces or even alloyed $\mathrm{In}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ QDs. Taking advantage of a series of samples in which photoluminescence (PL) spectra are characterized by the presence of a large number of distinct peaks corresponding to the emission from QD families having the same thicknesses $({h}_{\mathrm{QD}})$ in terms of an integer number of monolayers (MLs), we investigate intermixing by matching the experimentally observed transition energies to those calculated from tight-binding simulations. Two calculation frameworks are considered: (i) structures in which QDs are alloys with uniform P concentration [P] and (ii) nominally pure InAs QDs bordered by graded interfaces characterized by a diffusion length ${L}_{\mathrm{D}}$. Excellent agreement between theory and experiment is achieved with either framework. The analysis reveals that the two frameworks yield similar composition profiles as the composition at the center of the QD in the case of graded interfaces is modified to a point where it becomes essentially equal to that calculated for a QD of uniform composition. The calculations also indicate that in both frameworks, two substantially different solution sets are compatible with the experimental results. The first one is characterized by all QDs having the same P composition independent of their size, while the second solution shows a decreasing degree of intermixing with increasing ${h}_{\mathrm{QD}}$. To discriminate between the two solutions, we use them as input data in a Bloch-wave simulation of transmission electron microscopy (TEM) image contrast, providing a sequence of contrast versus ${h}_{\mathrm{QD}}$ values. Only the scheme in which the amount of [P] is the same for all QD ensembles is compatible with the TEM observations. From the analysis of an extensive array of CBE-deposited QD samples, it is concluded that the observed PL transitions can be attributed to a 3 ML thick wetting layer and 4--14 ML thick QDs, with [P] ranging from 6% to 10% depending on the growth conditions. The determined values of [P] and ${L}_{\mathrm{D}}$ suggest that surface $\mathrm{As}∕\mathrm{P}$ exchange and strain-driven alloying are the most probable mechanisms for substantial P incorporation.

Tuning spin properties of excitons in single CdTe quantum dots by annealing

Using polarization-resolved photoluminescence imaging in external magnetic fields we measurestatistically significant distributions of exchange splitting, diamagnetic shift and effectiveg-factor of excitons in tens of single CdTe quantum dots. Comparison between the as-grownand annealed structures shows strong suppression of ensemble inhomogeneities, withthe average exchange splitting reduced by half and significant narrowing of theg-factor distribution. Remarkably, the average value of the excitonicg-factor remains unchanged. This unique ability to yield highly uniform quantum dotensembles without hampering the exciton Zeeman splitting makes annealing ahighly attractive means for tuning the spin properties of quantum dot excitons.

Electrical Transport Phenomena in Systems of Semiconductor Quantum Dots

While a fairly good understanding of optical and transport properties that are associated with single quantum dots has emerged in recent years the understanding of the relation between these properties and the observed macroscopic optical and electrical properties of solid ensembles of such dots is still at a very rudimentary level. This is in particular so in regard to the transport properties where the interplay between inter-dot conduction and the connectivity of the dots network determines the macroscopic observations. Reviewing the basic concepts and issues associated with these two essential ingredients, and considering some recent experimental observations on quantum dot ensembles of CdSe and Si, an effort is made here to derive a whole-but-simple physical basis for the understanding of the transport and the optoelectronic properties of solid state ensemble of semiconductor quantum dots.

Gain Studies on Quantum-Dot Lasers With Temperature-Stable Emission Wavelength

In this paper, a comparative study of the gain spectra of quantum-well (QW) and quantum-dot (QD) lasers is presented. We point out the differences between the gain function of a QD laser and a QW laser and give a qualitative description of the effect that leads to the high wavelength stability of QD lasers. Furthermore, we demonstrate, by means of the gain spectra of an InGaAs and AlInGaAs QD laser, that devices with a high wavelength stability can be manufactured over a wide range of emission energies. The experimentally obtained data are fitted with a theoretical model that describes the gain of a QD ensemble. The characteristic features resulting in the high wavelength stability of QD lasers of 0.072 nm/K are analyzed and discussed.

Raman spectra of structures with CdTe-, ZnTe-, and CdSe-based quantum dots and their relation to the fabrication technology

Quantum-dot structures based on the CdTe, ZnTe, and CdSe semiconductors are prepared by molecular-beam epitaxy, colloid chemistry methods, and ball milling, and their Raman spectra are studied. Localized longitudinal phonons are observed in all spectra. The dependence of the localized phonon frequency on the thickness of the ZnTe barrier in CdTe/ZnTe quantum-dot superlattices is used to derive the dispersion relation for longitudinal phonons in ZnTe. The Raman spectra of ensembles of colloidal quantum dots differ from the spectra of the other objects by the absence of tellurium bands and a strong intensity of the longitudinal phonon band of CdTe. It is revealed that the spectra depend on the technology employed to prepare quantum-dot structures.

Negative characteristic temperature of long wavelength InAs∕AlGaInAs quantum dot lasers grown on InP substrates

InAs quantum dot lasers grown on (311)B InP substrates with AlGaInAs barriers have been fabricated and studied. A large decrease of the threshold current with temperature was observed from 110to140K. In the same temperature range, electroluminescence spectra showed a shape change, an energy shift with temperature, which cannot be fitted with a Varshni law, and a large decrease of the laser linewidth. These results can be related to a delayed thermalisation of carriers within quantum dot ensemble.

Effective one-dimensional electronic structure of InGaAs quantum dot molecules

Self-assembled InGaAs quantum dot molecules (QDMs) are grown by molecular beam epitaxy. Their optical characteristics at 77K have been characterized by photoluminescent (PL) measurements. The ground- and excited-state emissions of the quantum dot ensembles are explained by solving for a self-consistent solution of Schrödinger–Poisson equations in one dimension. Our simplified approach provides an effective cross-sectional structure of the QDM that predicts emission peaks with an accuracy of better than 1meV.

Long-Wavelength Emission at 1.5 m from InGaAs/GaAs Quantum Dots

Long-wavelength emission at 1.5 m at room temperature has been achieved from InGaAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapor deposition. The luminescence intensity of the InGaAs QDs was enhanced by optimizing the growth interruption duration without AsH3 supply just after the InGaAs QD layer had been formed. The use of InxGa1 xAs strain reducing layer to cover the InGaAs QDs shifted the QD emission wavelength to longer wavelength and made the PL wavelength reach 1.5 m at room temperature. The luminescence from the ground state and those from excited states were seen and this coexistence, and the change in the luminescence intensity with excitation power were explained by the coexistence of unoccupied ground states and excited states in di erent QDs that are electrically isolated from each other in a QD ensemble.

Quantum transport in semiconductor quantum dot superlattices: Electron-phonon resonances and polaron effects

Electron transport in periodic quantum dot arrays in the presence of interactions with phonons was investigated using the formalism of nonequilibrium Green's functions. The self-consistent Born approximation was used to model the self-energies. Its validity was checked by comparison with the results obtained by direct diagonalization of the Hamiltonian of interacting electrons and longitudinal optical phonons. The nature of charge transport at electron-phonon resonances was investigated in detail and contributions from scattering and coherent tunneling to the current were identified. It was found that at larger values of the structure period, the main peak in the current-field characteristics exhibits a doublet structure which was shown to be a transport signature of polaron effects. At smaller values of the period, electron-phonon resonances cause multiple peaks in the characteristics. A phenomenological model for treatment of nonuniformities of a realistic quantum dot ensemble was also introduced to estimate the influence of nonuniformities on current-field characteristics.

Optical properties of patterned InAs quantum dot ensembles grown on GaAs nanopyramids

We demonstrate the ability to form either coupled or isolated patterned quantum dot (PQD) ensembles on nanopatterned GaAs pyramidal buffers. The coupled PQD “clusters” consist of close-spaced PQDs with inter-QD spacing of 5nm. The isolated PQD “pairs” are comprised of two PQDs well separated by 110nm. The photoluminescence behavior, measured in integrated intensity, linewidth, and emission peak as a function of excitation intensity and temperature, indicates lateral coupling within the QD clusters and an isolated nature for QD pairs. The ability to tailor PQD formation and subsequent carrier recombination characteristic may prove useful in developing PQD-based devices for optical computing applications.

Optical transitions in polarized CdSe, CdSe∕ZnSe, and CdSe∕CdS∕ZnS quantum dots dispersed in various polar solvents

The optical transitions in ensembles of colloidal CdSe-based quantum dots (QDs) have been systematically studied as a function of the net QDs’ polarity/polarization and of the solvent’s polarity. While the general trend observed for all QD systems dispersed in different solvents is similar, the spectral shifts are more pronounced in core QDs than in core/shell structures. Our results can be rationalized by taking account of the electric field experienced by the QDs that results from their effective polarization in solvents of different polarities (quantum confined Stark effect) as well as from the effect of the external dielectric environment (solvatochromatic effect).

Correlation effect of Rabi oscillations of excitons in quantum dots

We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree–Fock-type Coulomb interactions between excitons.

IR-active vibrational modes of CdTe, CdSe, and CdTe/CdSe colloidal quantum dot ensembles

The frequencies of the vibrational modes of CdTe and CdSe quantum dots and CdTe/CdSe core/shell nanoparticles prepared by the colloidal chemistry method are determined using IR transmission and IR reflection spectroscopy. The experimental IR transmission spectrum of CdTe and CdSe nanocrystals exhibits a broad minimum located between the frequencies of the transverse (TO) and longitudinal (LO) optical phonons of bulk CdTe and CdSe crystals. The frequencies of the modes for ensembles of CdTe and CdSe quantum dots are considerably shifted toward lower frequencies as compared to those calculated for single quantum dots. This is explained by the dipole-dipole interaction between quantum dots. The frequencies of modes for the structures with core/shell nanoparticles differ little from the calculated frequencies. This suggests a weakening of the interaction in these structures due to the enhancement of dielectric screening.

Capacitance studies of multilayer ensembles of InAs QDs in a GaAs matrix

Quasi-static capacitance characteristics of multilayer arrays of vertically coupled InAs quantum dots (QDs) in a GaAs matrix were analyzed on the assumption of a Gaussian energy distribution of the ground state of the QDs. An array of InAs QDs with a characteristic base size of about 20 nm and height of ∼3 nm was ordered in the growth direction and had 3, 6, or 10 layers spaced by ∼5 nm. It was found that, as the number of layers increases from 3 to 10, the average binding energy of the ground electron state grows from ∼80 to ∼120 meV and the root mean square deviation characterizing the energy distribution of the levels of this state decreases from ∼30 to ∼15 meV.

Anisotropy of the electron Landé g factor in InAs/GaAs self-assembled quantum dots

We report on the precise determination of the electron Landé g factor and its anisotropy in self-assembled InAs/GaAs quantum dots (QDs), by means of measurements of Larmor precession frequencies in photoinduced circular dichroism (PCD) experiments performed with an oblique magnetic field. This optical technique, applied to an ensemble of QDs, is able to measure the electronic g factor corresponding to ground-state electrons resident in QDs emitting at a given energy of fundamental optical transition. In good agreement with recent theoretical results, we measure | g ⊥|=0.397±0.003 and | g ∥|=0.18±0.02 for QDs emitting at 1.32 eV.

Electron tunneling from quantum dots characterized by deep level transient spectroscopy

Electron tunneling from InAs∕GaAs quantum dots has been studied by deep level transient spectroscopy (DLTS). Comparing DLTS data with theory, we demonstrate how the results can be interpreted for situations where the emission mechanism is pure tunneling. An illusory anomalous tunneling dependence on electric field is resolved by taking into account the energy level distribution originating from size fluctuations in the quantum dot ensemble.

Localization of defects in InAs QD symmetric InGaAs/GaAs DWELL structures

The photoluminescence (PL), its power and temperature dependences have been studded for the ensembles of InAs quantum dots (QD) embedded in symmetric In 0.15Ga 0.85As/GaAs quantum wells prepared at different QD growth temperatures from the range 470–535 °C. The solution of the set of rate equations for exciton dynamics was used to analyze the nature of thermal activation energies of the QD PL quenching. It is revealed three different stages of thermally activated quenching of the QD PL intensity caused by thermal escape of excitons from the In 0.15Ga 0.85As/GaAs buffer and capping quantum wells (QWs) into the GaAs barrier and from the QDs into the GaAs barrier or into barrier/QW interfaces with their subsequent nonradiative recombination. The variety of activation energies of PL thermal quenching is discussed as well.

Optical characterization of thermally annealed self‐assembled ZnCdSe quantum dots

AbstractPost‐growth thermal annealing, (TA) in combination with mesa‐fabrication technologies, was applied to tune the shape and material composition of ZnCdSe quantum dots (QDs). The QDs were studied by means of photoluminescence spectroscopy. An enhancement of QD homogeneity due to TA is demonstrated by substantial narrowing of the QD emission, observed from large QD ensembles. Energy shifts of QD luminescence, exceeding 100 meV after thermal annealing, were observed. Tracing the evolution of single exciton lines, the activation energy of an enhanced diffusion process in small mesas was determined. Distinct changes of quantum dot symmetries were identified through a reduction of the exciton fine structure splitting. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)